Change Parameter Group of AWS RDS instance

See the "Modifying a DB Instance" section of the docs.

That page also goes over how to modify the db instance via the CLI, e.g.

aws rds modify-db-instance --db-instance-identifier <your instance name> --db-parameter-group-name <your PG name> 

See the docs for the other options you can specify in the modify-db-instance command.

To change anything like that you click on the RDS instance and then select "Modify". In the Modify screen you can select a different parameter group.

From @bitavoc's reply - If the dropdown doesn't let you select a parameter group recently created, check that both the parameter group and the RDS instance use the same major version of the engine.